In filtering methods and devices of the type concerned, a filterable fluid is passed through a column containing a magnetizable material of a porous structure, such as a magnetic grade stainless steel wool, the column being called a matrix. The matrix is placed under an external magnetic field sufficient in magnitude to effect magnetization and provides a large number of regions of very high magnetic field and magnetic field gradient along the paths of travel of the fluid to attract and retain the magnetic components therein.
The external magnetic field to the magnetic matrix may be produced with a permanent magnet constructed and arranged in a magnetic path with the matrix. It has been found, however, that the magnetic flux that a permanent magnet provides is most often insufficient to meet this end and further is reduced in magnitude and hence becomes ineffective as time of service elapses. Resort has therefore been had by the prior art to the use of an electromagnet energized by a continuous DC magnetization current. While an electromagnet is capable of producing a desirable magnet flux sufficient in magnitude, it has been found that it is extremeful wasteful of electric power and hence is quite low in efficiency.